1. Field of the Invention
The present invention relates to the field of compound bows including eccentrically mounted pulleys or wheels at the ends of the bow limbs, and more particularly to a modular wheel assembly for use with compound bows to permit varying of the draw weight and/or draw length of the bow.
2. Description of the Prior Art
Compound bows, as is well known in the art, utilize eccentrically positioned string or cable mounting pulleys secured to the free ends of the bow limbs. The pulleys provide a mechanical advantage to increase the amount of potential energy stored in the limbs as the arrow string is drawn. With this arrangement, when the arrow string is in the full draw position a maximum potential energy is stored in the bow while the force required to maintain the arrow in the full draw position is less than the maximum draw weight of the bow. As the arrow string portion is being drawn, the draw weight or string drawing force applied to the bow increases to a maximum draw weight and lets off to a lower draw weight at the full draw position. This has been found to substantially improve the performance of the bow and the ease of operation of the bow.
The physics of archery bow action is disclosed in my prior, co-pending patent application entitled "High Energy Limb Tip Cam Pulley Archery bow," filed Oct. 30, 1982, U.S. Ser. No. 438,204; which application is hereby incorporated by reference for this description of the physics of the bow action. This co-pending application is also incorporated by reference for other descriptions which it provides, as will be referenced hereinafter.
Perhaps the first description of the principles of the compound bow system is contained in U.S. Pat. No. 3,486,495, issued to Allen on Dec. 30, 1969. This patent discloses a fairly simple compound bow design in which a pair of one-piece, oblong pulley wheels are mounted at the opposite ends of the bow. As the arrow string is drawn back, the pulley wheels rotate and the draw weight first increases to a maximum and then decreases. Various other compound bow arrangements utilizing similar eccentric pulleys are described in U.S. Pat. Nos. 3,841,295; 3,854,467; 3,945,368; 3,948,551; 4,005,696; 4,054,118; 4,064,862; and 4,078,538.
The eccentric pulleys used with prior art units have generally been provided for using a single cable or string position on the bow. This single position is generally fixed and does not allow for cable adjustments. If it is desired to change the draw weight or the draw length of the compound bow, this is normally accomplished either by substituting different eccentric pulleys, or by physically changing the cable length, or by providing for adjustment between the limbs and the handle.
The draw length or the length at which the arrow string is pulled to impart potential energy in the limbs of the bow is one of the principal variables of the compound bow, and is determined by the physical requirements of the archer. For example, an archer of shorter height might prefer a draw length of 26 inches as compared to a taller archer who might prefer a draw length of 30 inches. However, in both cases the same percentage drop-off in draw weight may be desired between the maximum draw weight during the drawing cycle and the draw weight in the full draw position.
In order to effect a change in the draw length but maintain the same percentage drop-off it has been required in past devices to utilize pulleys having different diameters so as to provide a change in the length of the cable reeved about the pulley. By controlling the diameter size of the pulleys it has been possible to provide variations in the draw weight and draw length of the bow. Thus it has been the practice with conventional bows to change pulleys on the limbs to provide a preselected pulley diameter for a preselected draw length, requiring that a number of sets of pulleys of different diameters be made available for each bow. This practice substantially reduces the flexibility of a compound bow, for example in use as both a hunting and a competitive bow or for use by more than one archer of varying physical characteristics or requirements.
Another approach for enabling the draw length and/or draw weight to be adjusted has been to vary the path of the cable around or through the eccentric pulleys. In U.S. Pat. No. 4,241,715, issued to Jennings on Dec. 30, 1980, there is described a compound bow having eccentrics with various cable paths. In order to change the draw weight or length characteristics, the cable is threaded through the eccentrics along varying paths. A similar approach is described in U.S. Pat. No. 4,261,320, issued to Barna on Apr. 14, 1981. An adjustable compound bow is described in U.S. Pat. No. 3,958,551, issued to Ketchum on May 25, 1976, in which the eccentric pulley is provided in two halves, one associated with the take-up portion of the cable and the other with the bowstring portion. To vary the bow characteristics, different sized halves are combined to form an eccentric which is then mounted to the bow and threaded with the cable. In U.S. Pat. No. 4,061,124, issued to Groner, a limited adjustment in the draw length of the arrow string for the disclosed bow is accomplished by kinking portions of the cable to thereby limit the amount of cable that can be unwound from the pulley when the arrow string is drawn.
Another disclosure which may be relevant to the present invention is the design of the Bear Delta-V bow offered by Bear Archery of Gainesville, Fla. and as detailed in their 1983 Catalog "Bear Archery '83" on pages 4 and 5. The Delta-V bow incorporates conventional pulleys at the tips of the bow limbs and in this regard is significantly different than many of the other prior art references. Extending inwardly from the grip toward the bowstring are two, oppositely disposed draw length blocks incorporating special cam designs. However, in order to interface these cam designs with the remainder of the bow, a variety of added components are necessary thereby increasing cost and complexity. By using conventional pulleys at the tips, a number of stringing revisions are necessary. The interior/central location of the draw length blocks and cams is believed to be a disadvantage, note the need for power bumpers to avoid bowstring interference. Of possible interest with respect to the present invention is the removable cam portion (red part in catalog).
Although various methods and designs have been proposed in the prior art for adjusting draw length and weight, they have included at least two deficiencies. First, it has not typically been possible to independently adjust the draw length and the draw weight with such devices. Changes in diameter for the take-up track of the eccentrics produces a change in both the draw length and the draw weight. Second, all of the prior art units require that the compound bow be disassembled to some extent in order to permit the replacement of the eccentric wheels or portions thereof. Those in the art will recognize that it is preferable to enable adjustments of the draw length and draw weight without the need to unstring the bow, since this is a time-consuming and difficult procedure. For most, the requirement that the bow be unstrung means that the bow must be brought to a professional shop which has the equipment, such as a bow press, and the expertise to accomplish this task. Further, this restricts the speed and ease for making such changes and therefore limits the usefulness of the ability to adjust the bow characteristics.